Hey man! had the same problem but found out how to fix it:
If you type in the terminal "git branch" it shows you a list of all branches you have. The exercise will be complete if you have only your master branch left, so the simplest thing to do is to delete all other branches it lists. So in my case the terminal looked like this:
$ git branch
$ git branch -d new_branch
Deleted branch new_branch (some SAH).
Maybe this works for you aswell?